This invention relates to a turbofan engine, and more particularly, the invention relates to managing fan operability and operating characteristics.
A typical turbofan engine includes low and high spools housed within a core nacelle. The low spool supports a low pressure compressor and turbine, and the high spool supports a high pressure compressor and turbine. A fan or turbofan is coupled to the low spool. A fan nacelle surrounds the turbofan and core nacelle to provide a bypass flow path having a nozzle. Typically, the nozzle is a fixed structure providing a fixed nozzle exit area.
The fan's operating line must be controlled to avoid undesired conditions such as fan flutter, surge or stall. The fan operating line can be manipulated during engine operation to ensure that the fan operability margin is sufficient. The fan operating line is defined, for example, by characteristics including low spool speed, turbofan airflow and turbofan pressure ratio. Manipulating any one of these characteristics can change the fan operating line to meet the desired fan operability margin to avoid undesired conditions.
The engine is designed to meet the fan operability line and optimize the overall engine performance throughout the flight envelope. As a result, the engine design is compromised to accommodate various engine operating conditions that may occur during the flight envelope. For example, fuel consumption for some engine operating conditions may be less than desired in order to maintain the fan operating line with an adequate margin for all engine operating conditions. For example, fan operating characteristics are compromised, to varying degrees, from high Mach number operation to static conditions for fixed nozzle area turbofan engines. This creates design challenges and/or performance penalties to manage the operability requirements.
What is needed is a turbofan engine that provides improved operability for a variety of engine operating conditions while minimizing performance penalties throughout the flight envelope.